In angioplasty, mechanical techniques such as balloon expansion or stent implantation is likely to damage the blood vessel. In a site of lesion of the blood vessel, acute closure, caused by thrombosis, or re-stenosis caused by intimal hyperplasia of the blood vessel, as a curative reaction of the blood vessel wall, occurs frequently.
The acute closure is correlated with thrombosis. For its prevention, an anti-thrombotic therapy is performed by systemic medication, usually through a vein.
On the other hand, re-stenosis is caused by excessive hyperplasia of cells. At present, researches into drugs suppressing this hyperplasia of cells are advancing rapidly, and several drugs have demonstrated satisfactory results.
However, deleterious side effects have been pointed out because systemic medication at a high concentration or in a large quantity is required in order to achieve the effect of these drugs.
For this reason, an LDDS (local drug delivery system) recently has been used as a safe and effective method for prevention of acute closure or re-stenosis. Several such LDDS-based methods have been proposed for implanting a catheter in the blood vessel to introduce the drug to a target site. With these methods, it is necessary to keep the catheter inserted continuously over a prolonged time in the blood vessel, therefore the blood flow is interrupted, so that sufficient effect of the drug is difficult to achieve and consequently none of the methods has been put to practical use.
For this reason, it is a stent that is now stirring up attention as an LDDS member for transporting the drug to a target side in the blood vessel. By impregnating the stent with the drug, and by implanting the drug-impregnated stent into the target site, the medication can be administered locally. Since the stent is implanted and left at the target site in the blood vessel over a prolonged time without obstructing the blood flow, it can be used as an LDDS which guarantees sufficient pharmaceutical effect over a prolonged time period.
Meanwhile, the stent clinically used at present is almost formed of metal without exception.
With metal, it is only possible to deposit a drug on its surface, while it is not possible to impregnate the metal itself with the drug. Among the methods for depositing a drug on a metal stent, there are, for example, a coating method, a bonding method and a method of covering the stent with a polymer sheet impregnated with the drug. In case the drug is deposited on the metal stent by coating or bonding, there is presented a problem that the drug itself becomes peeled off from the stent surface. It is also difficult to have a quantity of the drug sufficient to manifest its pharmaceutical effect deposited on the stent surface.
With the method of covering the stent with a polymer sheet, the polymer sheet impregnated with a drug needs to be prepared at a high temperature, thus possibly detracting from the pharmaceutical effect of the drug.
In LDDS, it is necessary to control the content of the drug, the amount of released drug per unit time and the releasing time. In order to prevent acute closure or re-stenosis by LDDS more effectively, such control is desirable that the effective concentration of the drug at the target site in the blood vessel be maintained and that the drug be released for a predetermined time to the blood vessel and into the blood.